1. Technical Field
The present invention relates to an exposure device.
2. Description of the Related Art
Conventionally, in a semiconductor laser which emits light in the red to infrared region where the wavelengths are long, beam reshaping is carried out either by disposing a light beam limiting device (an aperture) after a coupling lens which converts the beam emitted from the laser crystal into substantially parallel light or converged light, or by limiting the light beam at the opening of the coupling lens.
In GaN semiconductor lasers, which emit blue light and which have been put into practical use in recent years, a material such as sapphire or SiC (silicon carbide) or the like which does not absorb blue light is used as the substrate.
Therefore, phenomena arise such as the reflected light reflected at the interior of the LD chip which emits the light becomes stray light and is discharged to the exterior, or returns to a vicinity of the active region and affects the state of oscillation of the laser, or the like. Therefore, problems arise in that the beam quality in the direction perpendicular to the PN coupling plane of the LD chip is poor, and a sufficient extinction ratio cannot be achieved. As a result, when this blue semiconductor laser is used as a light source for exposure, the image quality deteriorates. When this blue semiconductor laser is used as a light source for reading at a recording device or the like, it becomes a cause of erroneous detection.
Further, when this blue semiconductor laser is used as an exposure light surface of an image recording device which records an image by modulating the exposure intensity in accordance with an image signal, sufficient contrast cannot be obtained because the extinction ratio is insufficient.
As shown in FIG. 12A, a beam 110 emitted from an LD 102 is made into parallel light at a coupling lens 104, and is focused at a focal point 108 by a lens 106. Here, the stray light from the interior of the LD 102 becomes flare light 112. The beam quality in the direction orthogonal to the PN coupling plane of the LD 102 (i.e., in the direction of arrow V) deteriorates, and sufficient contrast cannot be achieved.
As shown in FIG. 12B, even if the flare light 112 is limited by inserting a slit formed on a plate 114 after the coupling lens 104, the object point of the flare light 112 is, differently from the light emitting point of the laser, not at the surface of the LD but rather at the interior of the LD. Therefore, the flare light 112 passes through the opening of the slit 114. It is difficult to eliminate only the flare light 112.
Moreover, as shown in FIG. 12C, a mechanism has been conceived of in which a portion of the beam is guided by a beam splitter 116 to a sensor 118 where the light amount is detected, and the driving voltage of the LD 102 is feedback-controlled at a controller 120. The light amount of the LD 102 is therefore monitored in real time in order to stabilize the light amount of the beam. However, at this time as well, flare light, which is not received at the light-receiving surface of the sensor 118 exists. Therefore, the relationship between the light amount at the image surface and the amount of light received at the sensor 118 is not linear, and the linearity thereof is lost.
Structures have conventionally been disclosed in which a diaphragm is fixed to the laser light source by an adhesive or the like, and the angle of the opening of the diaphragm with respect to the laser light source is fixed. (See, for example, Japanese Patent Application Laid-Open (JP-A) No. 11-58829 (page 1 and FIG. 1)). However, this is a structure which accurately maintains the angles between the positions of the light emitting points and the diaphragm opening when using an LD having plural light emitting points, and is not a countermeasure with respect to stray light.